Centrifugal impellers are widely used in combustion air blowers, in which they provide a large volume of air for combustion purposes. In these blowers, the impeller assembly is mounted in a generally disc-shaped casing which has an inlet opening adjacent the axis of the impeller and a discharge opening at the outer radius of the casing. Air or other combustion gas is drawn into the impeller, which imparts a high velocity to the air or gas, and the casing directs the flow through the discharge opening and into the combustion system.
When a centrifugal blower is operated in the pressure-vs.-volume range for which it was designed, pulsation, or "surge," is seldom a problem. There are, however, circumstances in which it is necesary to keep the blower running during periods of decreased demand, i.e., during periods of "turn-down" in the combustion system for which drastic decreases in fluid flow, by volume, are typical. Unfortunately, many centrifugal impellers known in the art "surge" in response to turn-down conditions in the system, and the surging can result in improper fluid flow, overheating, and consequential damage to combustion equipment.
Surging normally results during "turn-down" from the inability of an impeller to exert equal or greater pressure on the closed combustion system as is exerted during the higher volume of fluid flow characteristic of normal operations. Conversely, as long as there is no pressure drop at the blower during periods of reduced air delivery, no pulsation or surging will result. If pressure drops at the locus of the blower, however, the sequence known as surging begins. This surging may be visualized in contrast with and in terms of the normal operation of the impeller.
Under ordinary operating conditions, the pressure exerted by the blower equals the pressure in the overall combustion system, and fluid flow proceeds normally. If the pressure decreases at the blower, due to a turndown in fluid delivery, the pressure in the overall combustion system is momentarily greater than the pressure at the blower itself. Thus the air in the system tends to reverse its direction and flow back into the blower (this can be extremely dangerous in handling gas to a burner or combustion chamber) until both pressures become equalized. When this is achieved the blower again resumes its normal function of pumping air into the system--until the resistance in the system causes the sequence to repeat. It is this repetition which constitutes surging. The frequency and intensity of surging depends upon (1) the pressure-vs.-volume characteristics of the particular impeller, (2) the rate at which air is discharged from the system, and (3) the volume of the system into which the impeller is delivering air.
Previous attempts to minimize or eliminate surging concentrated on the operation of the impeller only within the pressure-vs.-volume range for which reduced impeller volume was unaccompanied by decreasd pressure. Implementation of such selective impeller operation usually required initial selection of an impeller which did not surge under conditions of the lowest anticipated turn-down but which also, as a result, usually did not operate with satisfactory volume capacity under ordinary combustion conditions. In addition, other attempts to eliminate surging provided for the use of inlet control dampers, blow-off valves and other auxiliary accessories which detracted from energy efficiency and provided, at best, a remedy for surging, not a preventive. As a result, a need persists for an impeller, for use in a combustion air blower, which demonstrates a surge point lower than the lowest anticipated turn-down and which preferably does not surge even under conditions of essentially 100% turn-down, in order to prevent surging, in the associated combustion system, before it occurs.